Abstract In recent years, increasing attention has been directed towards the use of B-cell depletion therapy as a treatment option in autoimmune disorders. It appears that the further development of these approaches will depend on a methodology to determine the relation of B-cell depletion to clinical response and how individual patients should be dosed. Thus far, patients have generally been followed by quantification of peripheral blood B cells; however, this measure clearly does not adequately reflect what is happening in the rest of the body. The ability to image B cells in multiple organs in vivo, repeatedly and with good spatial resolution and high sensitivity, would provide an excellent biomarker for this purpose. The overall goal of this proposal is to develop Gd-conjugated dendrimer nanoclusters as B-cell targeted T1 magnetic resonance (MR) imaging contrast agents for the quantification of B cells in lymphoid organs. Using monoclonal antibodies (mAb) to deplete B-cells, we will test these techniques in mice. Three specific aims are proposed: (1) Synthesize B cell- targeted, Gd-conjugated dendrimer nanoclusters (DNCs) with various physical and magnetic properties; (2) Evaluate the contrast enhancement, circulation time, biodistribution, and elimination of the various DNCs in mice; (3) Evaluate the ability to monitor B-cell depletion in mice.